Is the issue specific to Guest Introspection VM ? What version of VC and ESXI are we running. I have seen an issue with 6.5 VC and 6.0 host , no matter how much amount of resources are available Admission control never work unless we switch to Slot/Host failure tolerate policy in HA .

So if i understand correctly when Admission Control is enabled you not able to deploy and power on any agent VM's and it doesn't make any difference with other HA policy? Did you tried testing with some dummy vm's ? Are they experiencing same problem. Also do post cluster resource summary when Admission control is enabled .I hope you have tried re configuring HA as a break fix method ?

Is there any background or reason to stay with slot size admission control and configure specific fixed slot size?

No, Right now i'm abale to deploy the six VMs (GI & TrendMicro Deep Security) when I choose the % option. I want just to inderstand why I have this problem when I fix slot size to 2000Mhz or 3000Mhz with 16531Mb. Knowing that my GI VM has only 2CPU and 1024Mb of memory